Internal combustion engines typically include a plurality of pistons which are disposed within a plurality of corresponding combustion cylinders. Each of the pistons is pivotally connected to one end of a piston rod, which in turn is pivotally connected at the other end thereof with a common crankshaft. The relative axial displacement of each piston between a top dead center (TDC) position and a bottom dead center (BDC) position is determined by the angular orientation of the crank arm on the crankshaft with which each piston is connected.
A free piston internal combustion engine likewise includes a plurality of pistons which are reciprocally disposed in a plurality of corresponding combustion cylinders. However, the pistons are not interconnected with each other through the use of a crankshaft. Rather, each piston is typically rigidly connected with a plunger rod which is used to provide some type of work output. In a free piston engine with a hydraulic output, the plunger is used to pump hydraulic fluid which can be used for a particular application. Typically, the housing which defines the combustion cylinder also defines a hydraulic cylinder in which the plunger is disposed and an intermediate compression cylinder between the combustion cylinder and the hydraulic cylinder. The combustion cylinder has the largest inside diameter; the compression cylinder has an inside diameter which is smaller than the combustion cylinder; and the hydraulic cylinder has an inside diameter which is still yet smaller than the compression cylinder. A compression head which is attached to and carried by the plunger at a location between the piston head and plunger head has an outside diameter which is just slightly smaller than the inside diameter of the compression cylinder. A high pressure hydraulic accumulator which is fluidly connected with the hydraulic cylinder is pressurized through the reciprocating movement of the plunger during operation of the free piston engine. An additional hydraulic accumulator is selectively interconnected with the area in the compression cylinder to exert a relatively high axial pressure against the compression head and thereby move the piston head toward the top dead center position.
In a free piston engine as described above, the piston includes a piston head, a compression head and a plunger head which are commonly carried by a plunger rod and respectively disposed in the combustion cylinder, compression cylinder and hydraulic cylinder. The piston including the three separate heads is quite long, which increases the overall package size of the free piston engine. Moreover, as a result of the relatively large size of the piston, the mass of the piston is relatively heavy. The energy which is required for combustion of fuel within the combustion cylinder is related to the required kinetic energy of the piston when the piston is at a TDC position. The kinetic energy is a function of the mass and square of the velocity of the piston. Since the piston is relatively heavy, the piston is accelerated to a velocity which is relatively low in order to provide the kinetic energy needed for combustion. Moreover, since the piston is relatively heavy and the hydraulic fluid used to move the piston toward the TDC position is at a limited pressure, the acceleration of the piston is relatively slow and thus the stroke length is relatively long in order for the piston to reach the desired velocity. The slow acceleration, velocity and frequency of a conventional free piston engine results in a relatively low power output.
The present invention is directed to overcoming one or more of the problems as set forth above.